The present invention relates to a method and apparatus for the control of an ignition system spark timing for a spark ignition internal combustion engine, and more particularly to a method and apparatus for the control of an ignition system spark timing during cranking of a spark ignition internal combustion engine.
Description is now provided concerning the conventional art and the problems associated therewith. The spark timing control system for an internal combustion engine adjusts the timing of a spark to a certain degree before the top dead center (for example, 12.degree. BTDC/600 rpm) so as to cause the ignition to take place at the optimum instance to provide the maximum output because there is an ignition delay from the passage of a spark to the actual ignition of the fuel within the cylinder of the engine. The ignition delay in terms of time is substantially unchanged even if the engine revolution speed increases, but if it is considered in terms of crank angle, the ignition delay increases in accordance with an increase in the engine revolution speed because the piston speed increases in accordance with the increase in engine revolution speed. Thus, a centrifugal spark advance controller of the mechanical type, for example, is operatively connected to a distributor, thereby to advance the spark timing in accordance with an increase in the revolution speed because if the spark timing remains in the same level as that for the low revolution speed, an adequate output characteristic can not be provided when the revolution speed increases.
FIG. 1 is a graph showing one example of the spark advance characteristic provided by the conventional centrifugal advance controller, wherein the axis of abscissa designates engine revolution speed and the axis of ordinate designates spark advance value. Letter N.sub.1 designates a revolution speed beyond or above which spark advance value increases, letter N.sub.2 designates a revolution speed beyond or above which spark advance value becomes maximum, and letter N.sub.3 designates an idling speed, the solid line designates the spark advance characteristic when the engine temperature is within an ordinary-in-use range, and the broken line designates the spark advance characteristic when the engine is within a low temperature range. This is often called the "two-point method" which is characterized by selective use of two separate spark advance characteristics by switching in response to the low engine temperature range or the ordinary-in-use engine temperature range.
Induction vacuum as the engine is operating has a great influence on the ignition delay. Namely, since when the induction vacuum is great, i.e., when the throttle valve opening degree is small, the amount of air-fuel mixture inducted into a cylinder is little and the pressure within the cylinder is low even at the end of compression, thereby to cause a reduction in flame propagation, causing a drop in performance. As a countermeasure to this, it has been the conventional practice to provide a distributor with a vacuum spark advance controller.
FIG. 2 is a graph showing one example of a spark advance characteristic provided by the conventional vacuum advance controller, wherein the axis of abscissa designates induction vacuum, and the axis of ordinate designates spark advance value.
In the spark timing control system as mentioned above, the initial setting of spark timing is determined by means of mechanical position setting and the spark advance value is variable with respect to engine revolution speed and load, so that the spark advance during engine cranking can not help taking the same value as the spark advance value at idling. It has been a practice to use two-stage spark advance characteristic as shown by the solid line and the broken line in FIG. 1, but it has a problem in that switching is necessary in response to temperature, resulting in deterioration in manipulability and cost increase, and that the allowable choice is between two different values only.